This invention is directed towards a sound generation circuit, and more particularly to dual tone generation circuitry for use with a push button telephone set.
As shown in FIG. 2, a conventional push button dial telephone set 200 with holding tone includes a polarity coincident circuit 30 having sections 30a and 30b. An input section 44, which is connected to a telephone communication line (not shown), couples the telephone communication line to polarity coincident circuit 30. The output of section 30a is connected to a ringing tone generation circuit (i.e., ringer circuit) 41 which produces a signal supplied to a speaker 42 for producing a ringing tone. The output of section 30b is connected to the emitter of a transistor 31a and to one end of a switch 31c. The other end of switch 31c is connected to the base of a transistor 31b. Transistors 31a and 31b and switch 31c serve as a dial pulse sending switch 31. The emitter of transistor 31b is connected to a reference voltage such as ground. The collector of transistor 31b is connected to a base of transistor 31a. The collector of transistor 31a is connected to an integrated circuit for speech (hereinafter referred to as speech IC) 32. The output of speech IC 32 is connected to a receiver 43.
Telephone set 200 also includes a keyboard 33 connected as inputs to an integrated circuit for dialing (hereinafter referred to as dial IC 34). An integrated circuit for control of switch input and other functions (hereinafter referred to as control IC) 35 has two outputs. The first output of control IC 35 is connected as an input to dial IC 34. The other output of control IC 35 is connected as an input to an integrated circuit for holding tone (hereinafter referred to as melody IC) 36. One of two outputs of dial IC 34 and the output of melody IC 36 are supplied as inputs to a mixing circuit 37 (i.e., for mixing holding tone and dial tone signals together). The other output of dial IC 34 is connected to the base of transistor 31b. A low pass filter 38 filters out all but the low frequencies of the signal produced by mixing circuit 37. The output of low pass filter circuit 38 is supplied to speech IC 32.
Another output of speech IC 32 is supplied as an input to a loud speaker (or amplifier) circuit 39. A speaker 40 receives the output of circuit 39. Speech IC 32, dial IC 34, control IC 35, melody IC 36, mixing circuit 37, low pass filter 38 and loud speaker circuit 39 are each connected to the positive terminal of a voltage source V and to a reference voltage such as ground.
FIGS. 6A and 6B are block diagrams of melody IC 36 and dial IC 34. A more detailed description of all elements shown in FIGS. 6A and 6B, except a time setting circuit 100, is discussed below in connection with the present invention. Time setting circuit 100, shown in FIG. 6B, controls a timing dividing circuit 3. External circuitry (not shown) is also included within telephone set 200.
The number of parts required for construction of telephone set 200 prohibits its production at a reasonably moderate cost. As shown in FIGS. 6A and 6B, IC 36 and dial IC 34 include many of the same elements. For example, an oscillation circuit 1, a frequency dividing circuit 2, output frequency generation circuits 9 and 11, a digital/analog conversion circuit (hereinafter referred to as D/A circuit) 17 as well other elements are included in both melody IC 36 and dial IC 34. The duplicity of elements found in melody IC 36 and dial IC 34 complicates packaging of the sound generation circuit and results in conventional telephone set 200 having an unnecessarily high manufacturing cost.
Accordingly, it is desirable to provide a sound generation circuit having a simplified assembly, includes less parts and is less costly to manufacture than a conventional sound generation circuit.